As seen in FIG. 1, a conventional boiling water reactor has a reactor pressure vessel 10 and a core shroud 12 arranged concentrically in the RPV with an annular region 8, commonly referred to as the "downcomer annulus" therebetween. The core shroud 12 is a stainless steel cylinder surrounding the nuclear fuel core comprising a plurality of fuel bundle assemblies (not shown). Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate. During operation of the reactor, water is continuously recirculated down the downcomer annulus and then up through the core. This flow is induced by a multiplicity of jet pumps located in the downcomer annulus and driven by recirculation pumps (not shown) outside the reactor pressure vessel.
The core shroud 12 comprises a shroud head flange 12a for supporting the shroud head 22; a circular cylindrical upper shroud wall 12b having a top end welded to shroud head flange 12a; an annular top guide support ring 12c welded to the bottom end of upper shroud wall 12b; a circular cylindrical middle shroud wall comprising three sections 12d, 12e and 12f welded in series, with a top end of section 12d being welded to top guide support ring 12c; and an annular core plate support ring 12g welded to the bottom end of middle shroud wall section 12f and to the top end of a lower shroud wall 12h. The entire shroud is supported by a shroud support 14, which is welded to the bottom of lower shroud wall 12h, and by annular shroud support plate 16, which is welded at its inner diameter to shroud support 14 and at its outer diameter to RPV 10.
In the event of a seismic disturbance, it is conceivable that the ground motion will be translated into lateral deflection relative to the reactor pressure vessel of those portions of the shroud located at elevations above shroud support plate 16. Such deflections would normally be limited by acceptably low stresses on the shroud and its weldments. However, if the shroud weld zones have failed due to stress corrosion cracking, there is the risk of misalignment and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Stress corrosion cracking in the heat affected zone of any shroud girth seam welds diminishes the structural integrity of shroud 12, which vertically and horizontally supports the core top guide and the shroud head 22. In particular, a cracked shroud increases the risks posed by a loss-of-coolant accident (LOCA). During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head 22 and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant.
A repair method for vertically restraining a weakened core shroud utilizes tensioned tie rods 54 coupled to the shroud flange 12a and to the shroud support plate 16, as seen in FIG. 1. The lower end of the tie rod/lower spring assembly hooks underneath a clevis pin 20 inserted in a hole machined into gusset plate 18, which plate is in turn welded to shroud support plate 16 and RPV 10. In addition, the shroud 12 is restrained laterally by installation of wishbone springs 56 and 72, which are components of the shroud repair assembly.
Referring to FIG. 1, the shroud restraint tie rod/lower spring assembly comprises a tie rod 54 having a circular cross section. A lower end of tie rod 54 is anchored in a threaded bore formed in the end of a spring arm 56a of lower spring 56. Tie rod 54 extends from the end of spring arm 56a to a position adjacent the outer circumferential surface of the top guide support ring 12c. The upper end of tie rod 54 has a threaded portion.
The lower spring 56 is anchored to a gusset plate 18 attached to the shroud support plate 16. The lower spring 56 has a slotted end which straddles gusset plate 18 (see FIG. 3) and forms a clevis hook 56c. The clevis hooks under opposite ends of a clevis pin 20 inserted through a hole machined in gusset plate 18. Engagement of the slotted end 56c with the gusset plate 18 maintains alignment of lower spring 56 under the action of seismic motion of the shroud, which may be oblique to the spring's radial orientation.
The tie rod 54 is supported at its top end by an upper support assembly 62 which hangs on the shroud flange 12a. A pair of notches or slots are machined in the shroud head ring 22a of shroud head 22. The notches are positioned in alignment with a pair of bolted upper support plate segments 64 of upper support assembly 62 when the shroud head 22 is properly seated on the top surface of shroud flange 12a. These notches facilitate coupling of the tie rod assembly to the shroud flange.
The pair of notches at each tie rod azimuthal position receive respective hook portions 64a of the upper Support plates 64. Each hook 64a conforms to the shape of the top surface of shroud flange 12a and the shape of the steam dam 24. The distal end of hook 64a hooks on the inner circumference of shroud dam 24.
The upper support plates 64 are connected in parallel by a top support bracket (not shown) and a support block 66 which forms the anchor point for the top of the tie rod. Support block 66 has an unthreaded bore, tapered at both ends, which receives the upper end of tie rod 54. After the upper end of tie rod 54 is passed through the bore, a threaded nut 70 is torqued onto the upper threaded portion of the tie rod 54.
As seen in FIG. 1, the assembly comprised of support plates 64 with hooks 64a, support block 66, tie rod 54, lower spring 56, clevis pin 20 and gusset plate 18 form a vertical load path by which the shroud flange 12a is connected to the shroud support plate 16. In the tensioned state, the upper support plates 64 exert a restraining force on the top surface of shroud flange 12a which opposes separation of the shroud 12 at any assumed failed circumferential weld location.
Lateral restraint at the elevation of the top guide support ring 12c is provided by an upper spring 72 having a double cantilever "wishbone" design. The end of the radially outer arm of upper spring 72 has an upper contact spacer 74 rotatably mounted thereon which bears against the inner surface of the wall of RPV 10.
Spring arm 56a of lower spring 56 laterally supports the shroud 12 at the core plate support ring 12g, against the vessel 10, via a lower contact spacer 76. The top end of spring arm 56a has a threaded bore to provide the attachment for the threaded bottom end (not shown) of tie rod 54. The member 56d connecting the wishbone spring arms 56a, 56b to clevis hook 56c is offset from the line of action between the lower end of tie rod 54 and clevis pin 20 to provide a vertical spring compliance in the load path to the tie rod. A middle support 80 is preloaded against the vessel wall at assembly by radial interference which bends the tie rod 54, thereby providing improved resistance to vibratory excitation failure of the tie rod.
During installation of the shroud repair hardware shown in FIG. 1, the tie rod/lower spring assembly comprising tie rod 54 screwed into lower spring 56 is suspended from a cable and lowered into the annulus. During its descent, the assembly must be carefully maneuvered past various obstacles without damaging internal reactor components. Ultimately, the assembly is positioned so that it hangs plumb over the gusset plate. At this juncture, the tie rod/lower spring assembly must be maneuvered so that the clevis hook is hooked underneath the clevis pin on the gusset plate, as seen in FIG. 3. To accomplish this, the clevis hook at the bottom of the suspended assembly must be displaced radially inward in the downcomer annulus until there is radial clearance vis-a-vis the clevis pin. With the clevis hook in this radially inwardly displaced position, the tie rod assembly is lowered a few inches until the tip of the clevis hook clears the bottom of the clevis pin. Then the force displacing the bottom end of the suspended tie rod assembly radially inward is removed, allowing the lower spring clevis 56c to "drift" under the clevis pin 20. The tie rod assembly is now properly positioned and simply lifted up to engage the clevis pin in the clevis hook. After clevis hook 56c has been hooked under clevis pin 20, the lower end of the tie rod assembly is braced in the hooked position and the upper end of the tie rod assembly is uncoupled from the hoisting cable to allow the upper support assembly 62 to be installed, followed by upper spring 72.